1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive using an optical sensor to detect a position of an actuator arm.
2. Description of the Prior Art
FIG. 1 shows an exploded view of a prior art disk drive comprising a disk 2 rotated by a spindle motor 4, and a head 6 coupled to a distal end of an actuator arm 8 which is rotated about a pivot 10 by a voice coil motor (VCM) in order to actuate the head 6 over the disk 2. The disk 2, spindle motor 4, head 6, actuator arm 8, and VCM are enclosed in a head disk assembly (HDA) comprising a base 9 and a cover 11. The VCM comprises a voice coil 12 coupled to the base of the actuator arm 8 and one or more permanent magnets attached to a yoke 14. When the voice coil 12 is energized with current, the resulting magnetic flux interacts with the magnetic flux of the permanent magnets to generate a torque that rotates the actuator arm 8 about the pivot 10. A tang 16 attached to the actuator arm 8 interacts with a crash stop 18 to limit the stroke of the actuator arm 8, and also provides a latching mechanism (e.g., using a magnet) to maintain the actuator arm 8 in a latched position while the disk drive is powered down. Alternatively, the actuator arm 8 may be parked on a ramp located at the outer periphery of the disk 2 when the disk drive is powered down.
The disk 2 typically comprises embedded servo sectors recorded at a periodic interval around the disk 2 which provide coarse position information (e.g., a track address) used to seek the head 6 to a target track, and fine positioning information (e.g., servo bursts) used to maintain the head 6 over the target track during read/write operations. Control circuitry within the disk drive processes the position information detected from the servo sectors to implement a position controlled servo system during normal operation. During transition modes, such as unlatch or ramp-load during power on and latch or ramp-unload during a power failure, the position information is unavailable from the servo sectors. For these modes, prior art disk drives typically process the back EMF voltage generated by the VCM to estimate a velocity of the actuator arm 8, and the control circuitry processes the estimated velocity to implement a velocity controlled servo system. However, estimating the velocity of the actuator arm 8 by evaluating the back EMF voltage requires expensive analog circuitry which can typically provide only a crude estimate due to the mechanical variations of the VCM (e.g., the resistance of the voice coil 12). In addition, the back EMF voltage is typically very small at low speed requiring low driving voltages and undesirable inefficiencies, particularly if the driving voltage is generated from the momentum of the spindle motor 4 during a power failure.
There is, therefore, a need to detect the position (and/or velocity) of an actuator arm in a disk drive more effectively, for example, during transition modes (e.g., power on/off, etc.) when the position information from the embedded servo sectors is unavailable.